Long-term effects of simulated gastric juice alternated with brushing on hardness, substance loss, flexural strength and reliability of CAD-CAM monolithic materials

Abstract Objectives: The purpose of this study is to evaluate, over a simulated 5-year period, the effect of simulated gastric juice alternated with brushing on CAD-CAM monolithic materials considering microhardness, substance loss, flexural strength, and reliability of the materials. Methodology: Blocks from Lava Ultimate (LU), Vita Enamic (VE), IPS Empress CAD (EMP), IPS e.max CAD (EMAX), and Vita Suprinity (VS) were milled into cylinders and sliced into disks. The EMAX and VS were crystallized, and all specimens were polished with silicon carbide papers and allocated as follows: 1) artificial saliva + brushing or 2) simulated gastric juice (0.113% hydrochloric acid (HCl) solution in deionized water, pH 1.2) + brushing, simulating 1, 3, and 5 years of clinical function. Each year of clinical function was simulated by three repetitions of immersion for 3 hours in artificial saliva or simulated gastric juice followed by 1,217 brushing cycles. The microhardness and substance loss were evaluated at baseline (T0) and at each year by using a Vickers hardness tester and an analytical balance. The biaxial flexural strength (BFS) test was performed in a mechanical testing machine at the end of the 5th year. Weibull modulus was calculated from the BFS data. Results: The microhardness of the LU was not influenced by the treatment, whereas that of the other materials, in certain years, was significantly lower in the gastric juice + brushing groups in comparison with artificial saliva + brushing groups. In general, the materials did not present a significant change in microhardness over time, for either of the treatments. The LU alone showed greater substance loss in the gastric juice + brushing groups for every year. In both treatments, the LU, VE, and EMP exhibited a significant increase in the substance loss over time. The treatment did not affect the BFS of the materials. The gastric juice + brushing decreased the reliability of the VE. Conclusions: All materials were somehow impaired by the gastric juice + brushing in at least one of the evaluated parameters, except for the BFS. However, in a deeper analysis, the LU would be the least indicated materials, followed by VE, for patients with eating disorders.


Introduction
The possibility of fabricating satisfactory restorations of a single-session application, regarding several aspects, 1  However, since they are monolithic, these materials are exposed to the oral environment, under the action not only of extrinsic factors, such as acids from food, but also of intrinsic factors, among which the gastric juice can be highlighted in patients with bulimia nervosa or gastroesophageal reflux, the prevalence of which has been increasing due to bariatric surgeries. 9 Despite the strong scientific and clinical evidence supporting the adverse effects of the gastric juice on both the tooth structure 10,11 and restorative materials, like direct and indirect composite resins, 12,13 few studies have investigated the impact of this acidic challenge on CAD/CAM monolithic materials.

Methodology Specimen preparation and treatments
The following materials were tested: Lava Ultimate

Substance loss
For the substance loss (μg), 20 additional disks were obtained from each material, being 10 for each treatment. The readings at the baseline (T 0 ) and at the end of the 1 st (T 1 ), 3 rd (T 3 ), and 5 th (T 5 ) year were made in the same specimens. Before the readings, the specimens were ultrasonically cleaned in distilled water for 5 minutes and stored in an oven at 37°C for 7 days. For the weighing, the specimens were positioned

Material and Lot numbers
in which v is the Poisson's ratio (0.25), r1 is the radius of support circle, r2 is the radius of loaded area, and r3 is the radius of the specimen.

Statistical Analysis
Microhardness data and differences in mass (ΔT 0 -T 1 , Mauchly's sphericity for the differences in mass was not met, the lower limit was used to calculate the Epsilon correction factor (ε=0.5). The Bonferroni test was used to assess differences among groups. The BFS data were submitted to 2-way ANOVA with material and treatment as independent variables. Normality and homoscedasticity were verified by Shapiro-Wilk (P>0.178) and Levene (P<0.001) tests. Although the assumption of homoscedasticity was not met, we decided to proceed with the ANOVA, which is known to be robust for moderate deviations from normality and homoscedasticity in cases of block design with balanced groups, 34 and the Games-Howell test was employed to assess the differences among groups.
Weibull modulus was calculated from the BFS data.
The level of significance was set at 0.05. The statistical analyses were performed using the IBM SPSS Statistics v22.0 statistical software.

Cycles
Simulation Correspondence         This finding can be corroborated by Kulkarni,et al. 20 (2020) who did not observe a significant difference in the BFS when the EMAX was only brushed or exposed to hydrochloric acid and subsequently brushed. No other study with a similar protocol was found for comparison.
In our study, the BFS was strongly determined by the material, as revealed by the effect size analysis. The higher strength of the glass-ceramics EMAX and VS in comparison with the other materials evaluated in this study is already known. 2 17 (2018) reported that the LU exhibited greater strength than the VE, even after immersion in hydrochloric acid; in their study, however, brushing was not performed, which could change the strength relationship between these two materials. The higher reliability of the LU in relation to the ceramic materials, as well as its lower reliability in relation to the VE can be corroborated by Finally, the gastric juice + brushing decreased the reliability of the VE. In a deeper analysis, the LU would be the least indicated materials, followed by VE, for patients with eating disorders.